when apply do work energy principal theory

hi Radha krishna,

v use work-energy theorem on a system on which there are no external forces applied (or) when the external forces wch are applied on the system nullify each other so that the net external forces on the system will be zero.....

Approved

It states that the total amount of energy in an isolated system remains constant over time. The total energy is said to be conserved over time. For an isolated system, this law means that energy can change its location within the system, and that it can change form within the system, for instance chemical energy can become kinetic energy, but that energy can be neither created nor destroyed. In the nineteenth century, mass and energy were considered as being of quite different natures.